Thermometer immersion effect

A number of errors in the mercury-in-glass thermometer need to be considered. Most are connected with irreversible or slowly reversing bulb contractions and are coupled with time and immersion effects. From the differential expansivity of mercury and glass, one can calculate that the volume within the capillary corresponding to one kelvin on the scale of the thermometer must communicate with 6,000 times this volume in the bulb of the thermometer to register the proper change of temperature. The most important limitation of the absolute accuracy of a thermometer, thus, resides in the precision to which the volume of the bulb can be maintained. 

The immersion effect is the largest and most common of all the causes of error, one which always must be corrected for. The drawing on the left in Fig. 4.4 illustrates a thermometer immersed only partially and the use of the correction equation. A length of n degrees on the scale of the mercury column is outside of the bath. The... 

Liquid in Glass Thermometers. Mercury-in-glass thermometers (or better yet, mercury-in quartz) function well between -25°C and 360°C their typical precision is 0.1 K. They must be corrected for (1) relatively small pressure effects and (2) a relatively large "exposed-stem correction," due to the different coefficients of thermal expansion of mercury and glass in the part of the thermometer not immersed in the system being measured. 

WET-BULB TEMPERATURE. The wet-bulb temperature is the steady-state, non-equilibrium temperature reached by a small mass of liquid immersed under adiabatic conditions in a continuous stream of gas. The mass of the liquid is so small in comparison with the gas phase that there is only a negligible change in the properties of the gas, and the effect of the process is confined to the liquid. The method of measuring the wet-bulb temperature is shown in Fig. 23.4. A thermometer, or an equivalent temperature-measuring device such as a thermocouple, is covered by a wick, which is saturated with pure liquid and immersed in a stream of gas having a definite temperature T and humidity ff. Assume that initially the temperature of the liquid is about that of the gas. Since the gas is not saturated, liquid evaporates, and because the process is adiabatic, the latent heat is supplied at first by cooling the liquid. As the temperature of the liquid decreases below that of the gas, sensible heat is transferred to the liquid. Ultimately a steady... 

The difference between the pressure measured in the vapor-pressure bulb and the controlled surface vapor pressure is AP and 0.030 in. Hg corresponds to a temperature difference of one millidegree. The immersion depth of the vapor-pressure bulb in liquid helium is /i. At corresponding depths in the liquid helium, there is little difference in the pressures indicated by the vacuum-jacketed and non- jacketed vapor-pressure thermometers. These results indicate that the effect of a cold spot on the vapor pressure thermometer at 4.2 K is negligible. Just above the point, however, effects of cold spots have been observed. This type of bulb eliminates the effect. 

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